Fan and high temperature limit control for warm air furnaces

ABSTRACT

The control device has a helical bimetal element, a fan switch, a high temperature limit switch, and a single rotary cam operatively connecting the bimetal element and switches. Low friction bearing means for journalling the cam, convenient means for calibrating, and novel fan switch adjustment means to vary the automatic operation or to effect continuous operation of the fan are other salient features of the device.

United States Patent 1191 1111 3,829,811 Miller Aug. 13, 1974 [54] FAN AND HIGH TEMPERATURE LIMIT 2,424,250 7/1947 Morris 337/351 x CONTROL FOR WARM AIR FURNACES 2,435,004 1/1948 Healey 337/351 2 2,833,892 5/1958 Payne 337/353 David E. Miller, Waterloo, 111.

Assignee: Emerson Electric Co., St. Louis, Mo.

Filed: July 19, 1973 Appl, No.: 380,552

Inventor:

US. Cl 337/353, 337/351, 337/352 Int. Cl. HOlh 37/52 Field of Search 337/14, 16, 42, 43, 44,

References Cited UNITED STATES PATENTS 8/1931 Lindeman ct a1 337/353 5/1939 Shaw 337/352 4/1943 Malone 337/353 X Primary Examinerl. D. Miller Assistant E.\'aminerFred E. Bell Attorney, Agent, or Firm-Charles E. Markham [57] ABSTRACT 14 Claims, 15 Drawing Figures PlTENTEflauclsmu 3.829.811

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This invention relates to combination fan and high temperature limitcontrol devices employed in circulating warm air heating systems and particularly to a device of this kind in which a circulating fan switch and a high temperature limit switch are operated by a single helically-wound bimetal element.

HelicalIy-wound bimetal thermostatic elements such as disclosed in US. Pat. No. 2,588,688, which impart rotary movement for the operation of a control switch or switches through a coaxial torsion rod in response to temperature changes, have been in use for many years and the advantages of this type of thermostat are wellknown and understood.

When operation of a control switch is effected by multiplied angular movement of the torsion rod, as in the aforementioned patent disclosure, a slight lateral movement of the torsion rod does not significantly affect the control point at which the switch is operated.

Further objects and advantages will become apparent when reading the following description in connection with the accompanying drawings.

In the drawings FIG. 1 is a side elevational view of a thermostatically operated switching device constructed in accordance with the present invention;

FIG. 2 is a front elevational view of the device shown in FIG. 1;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 1;

FIG. 4 is an interior elevational view of one of the casing sections including the switch operating cam and helical bimetal element. This view is taken along line 4-4 of FIGS. 2 and 3.

FIGS. 5 and 6 are interior elevational views of the g two identical half sections of the casing, each including The joumalling of the free end of the torsion rod does i not, therefore, pose a problem. That is to say, the torsion rod need not be so closely fitted in a bearing as to likely result in some variable frictional forces which would cause a variation of the control point.

On the other hand, when operation of the switch or switches is to be effected by radial movement imparted through a rotary cam mounted on the torsion rod, any slight lateral movement of the torsion rod will significantly affect the control point so that the cam must be held closely to rotation about an axis. It will be appreciated that even though perfect axial alignment of the torsion rod and bimetal helix could always be achieved in manufacture some lateral force component is likely to be applied to the torsion rod as the helix warps in response to temperature change.

It is, therefore, one object of this invention to provide a generally new and improved switching device having a helically-wound bimetal element including a torsion rod and a radial cam thereon forming an operative connection with a control switch and including novel and substantially frictionless means for precisely joumalling the cam against lateral movement.

Another object is to provide a thermostatically operated switching device having a helically-wound bimetal element includinga torsion rod and a single radial cam thereon forming an operative connection with two control switches.

Another object is to provide a thermostatically oper ated switching device, as in the preceding paragraph, in which the single radial cam engages the switches and effects the closing of one switch and the opening of the other when the cam is rotated sufficiently in one direction and effects the opening of the one and the closing of the other when the cam is rotated oppositely.

Another object is to provide a thermostatically operated control device for controlling the operation of a circulating fan and for limiting the furnace plenum temperature in a warm air heating system comprising a casing enclosing and supporting a pair of control switches and a cam arranged to be rotated by a thermostatic element to operate the switches, in which the casing comprises two identical half sections each mounting an identical switch and jointly providing means for mounting the thermostatic element and for supporting the cam for rotation therein.

a switch and conductive terminal bars, with the half section shown in FIG. 6 being rotated with respect to the half section shown in FIG. 5.

FIG. 7 is an enlarged fragmentary crosssectional view taken along line 7-7 of FIG. 4;

FIG. 8 is a cross-sectional view through the fan control adjustment lever and is taken along line 88 of FIG. 1;

FIG. 9 is a cross-sectional view of the operating differential adjustment screw taken along line 99 of FIG. 1;

FIGS. 10 and 11 are side elevational and longitudinal cross-sectional views, respectively, of the snap action switch blade;

FIGS. 12 and 13 are side and end elevational views, respectively, of the conical bearing member; and

FIGS. 14 and '15 are side and end elevational views, respectively, of the cam.

Referring to the drawings in more detail, the device comprises a casing consisting of two identical elongated rectangular casing sections generally indicated at 10 and 12, see FIGS. 5 and 6. The casing section 12 is shown rotated end for end with respect to casing section 10. The casing sections are placed together in these relative positions when assembled and are held together by rivets 11 passing through holes 13 in the bottom walls l4thereof. The casing sections each have,

in addition to a bottom wall 14, side walls 16 and 18.

End wall portions 20 and 22 extend perpendicularly from the ends of side wall 16, and end wall portions 24 and 26 extend perpendicularly from the ends of side wall 18. The end wall portions 24 and 26 are spaced outward from end wall portions 20 and 22 and extend in overlapping relationship therewith. When the casing sections are assembled, a central chamber 28 and end chambers 30 and 32 are formed, see FIG. 3. Spaces between the ends of wall portions 20 and 22 and side wall 18 provide communication between the central chamber 28 and end chambers 30 and 32. The end chambers 30 and 32 are open at the front, as shown in FIG. 2.

The casing half sections each include an identical snap switch. The switches are identically arranged and mounted in their respective casing sections. Each switch comprises an elongated flexible switch blade, indicated at 34, of conductive material, see FIGS. 10 and 11. Each of the blades 34 is anchored atone end to a boss within chamber 28 by a rivet 36 which passes through a hole 37 near one end of the blade. Blades 34 each have a movable contact 38 at the opposite free end thereof. Blades 34 are further provided with a U- shaped cutout portion 39 which provides two parallel tension legs 40 and a parallel central compression leg 42 having a free end. The blades 34 also have a second cutout portion 44 near the anchored end thereof.

Overlying the anchored end of each of blades 34 is a flat conductive strip 46 which is also anchored to the casing by the rivet 36. The conductive members 46 have a U-formed portion 47, see FIG. 3, which extends perpendicular to the blade 34 through its cutout portion 44, then extends parallel in spaced relationship with the blade, and then extends perpendicular toward the blade through cutout portion 39 and terminates in a notched end 49 which receives the free end of the blade compression leg 42. The conductive strip 46 also includes a terminal lead portion extending from anchor 36 through the gap between the end of wall 22 and side wall 18 and into one or the other of the open end chambers 30 and 32, and it terminates therein in a reduced perforated end portion 48 for connection to an external circuit lead.

Each snap-action switch further includes a stationary contact 50 mounted on a transversely extending portion 51 of a second, flat, conductive strip 52 which overlies the free end of switch blade 34 and its movable contact 38. Conductive strip 52 is anchored to internal bosses by a pair of rivets 54. The strip 52 further includes a portion extending through the gap at the end of wall portion 20 into one of the open chambers 30 or 32 and terminates therein in a reduced perforated end portion 56 for connection to an external circuit lead.

Casing sections and 12 are provided with identical cavities 58 in the end faces of side wall 16, each cavity being adapted to receive one half of a conical thrustbearing member 60, shown in FIGS. 12 and 13. Casing sections 10 and 12 are also provided with identical semicylindrical cavities 62 in the end faces of opposite side wall 18. Each cavity 62 has a portion 63 forming one half of a round bore adapted to receive a mounting tube 64. Each cavity 62 also has a portion 67 forming one half of an annular groove.

The mounting tube 64 is slit longitudinally the length thereof at 66 and is of such material and of such wall thickness as to be compressible for insertion into the bore formed by the cavity portions 63. The bore formed by cavity portions 63 is smaller in diameter than the free uncompressed diameter of mounting tube 64, so that the tube when released after insertion into the bore in a compressed condition is held in the bore against rotation by a considerable frictional force. The mounting tube 64 is further provided with several circularly spaced projections 68 near the inner end thereof, see FIGS. 4 and 7. These projections 68, which are preferably formed as outwardly extruded portions of the tube wall, are entered into the annular groove jointly formed by the cavity portions 67, whereby the mounting tube 64 is restrained from outward axial movement.

The outer end of the mounting tube 64 is connected to the inner end of a helically wound bimetal strip 72 by rivets 74, see FIG. 1. A torsion rod 76, preferably of non-circular cross section, is connected at its outer end to the outer end of bimetal helix 72 by rivets 73 and extends therefrom coaxially through the bimetal helix through the mounting tube 64 and into a non-circular bore 78 in one end of a cam member 80, see FIGS. 4, 14, and 15.

The cam member 80 has a cylindrical portion 81 and a reduced cylindrical portion 82 at one end which has a blind non-circular axial bore 78 therein receiving the free end of a non-circular torsion rod 76. At its opposite end cam 80 has an axial conical recess 84 formed therein in alignment with bore 78. Conical recess 84 1 receives the conical end of the thrust bearing 60. The

cam 80 is further provided with a peripheral cam surface 85 which extends circumferentially around the cylindrical portion 81 somewhat more than 180. The cam surface 85 is substantially symmetrical about a center line bisecting it and comprises camming surfaces 83 and 89 joined by an intermediate dwell surface 87, the rise and fall of camming surface 83 being opposed to that of camming surface 89.

Referring to FIG. 3, the snap switches 33 are shown in a closed contact position with the bead portions 86 of switch blade 34 engaged by the camming surfaces 83 and 89. When sufficient outward lateral pressure and movement is imparted to the bead 86 of either of the blades 34 by rotation of cam 80, the blade will snap to an open contact position and into engagement with the end of an adjacent differential adjusting screw 88. The bimetal helix is formed with the high expansion laminate on the inside, so that, with reference to FIG. 3, when the temperature to which it is responsive rises, cam 80 will be rotated counterclockwise. The left-hand switch in casing 10 will therefore be operated to an open contact position when cam 80 is rotated counterclockwise a predetermined amount from the position shown in response to a temperature increase.

When this occurs, the right-hand switch in casing 12 will not, however, be affected by this counterclockwise cam rotation because the radius of the cylindrical portion 81 is less than any portion of cam surface 85. However, when the cam 80 is rotated in a clockwise direction a predetermined amount from the position shown in response to a decrease in temperature, the righthand switch in casing section 12 will be operated to an open contact position. This clockwise rotation of the cam 80 from the position shown will also, therefore, have no effect upon the left-hand switch in casing section 10.

The amount of outward lateral movement required to be imparted to the switch blade 34 to effect operation from a closed to open contact position may be varied by laterally moving the notched end 49 of conductive member 46. Lateral movement of the notched end 49 of conductive member 46 varies the stress on the blade compression leg 42. Means to accomplish lateral positioning of notched end 49 is provided in the form of an adjusting screw 90 threadedly engaged in conductive member 46 near the notched end 49 thereof. The adjustment screw 90 passes through a hole in the bottom wall of the casing and has a knurled or serrated head 92 adapted to selectively receive an operating handle 93,.see FIG-8. As screw 90 is rotated in a direction to bend the end of conductive member 46 outward and move the notched end 49 outward from cam 80, a greater outward movement of the blade 34 at head 86, and, consequently, greater rotation of cam 80, is required to effect snap-action operation of the switch to an open contact position.

The amount of lateral inward movement of the switch blade at bead 86, and therefore the amount of rotation of cam required to permit the switch to return from an open contact to a closed contact position,

may be varied by rotation of the differential adjustment screw 88. As adjustment screw 88 is turned outward to further space the end thereof from the stationary contact 50, an increased inward movement of the blade bead 86 and, therefore, rotation of cam 80 is required to permit the switch to return to its closed contact position. The heads of differential adjustment screws 88 are provided with screwdriver slots and may selectively include a knurled turning knob 94, see FIGS. 1 and 9.

When assembling the casing sections and 12, the conical thrust bearing 60 is entered into the cavity 58 of one of the casing sections. A sub-assembly comprising the mounting tube 64, the bimetal helix 72, and the torsion rod 76 with the cam member 80 mounted thereon is next assembled in the same casing section, with the mounting tube projections 68 entered into the recess portion 67, see FIG. 4. The axial dimension from the outward radial surface 71 of the recess portion 67 to the end of conical bearing member 60 is less than the free dimension from the outward radial surfaces of projections 68 to the bottom of conical recess 84 in cam 80. It is necessary, therefore, to stretch the helical bimetal element axially somewhat to permit entry of projections 68 into recess portion 67. As a result of this stretching, a constant resilient axial force biases the bottom of conical cam recess 84 against the end of conical thrust bearing 60 when the bimetal helix is released.

It will be seen in the arrangement described that the cam 80 and therefore the free end of the torsion rod 76 are freely journalled with negligible friction precisely on an axis extending through the bearing member 60, the cam 80, and the torsion rod 76. Any misalignment of the outer connected ends of the helix and torsion rod with this axis would merely cause the cam to pivot slightly about the very small areas of the bearing contact surfaces. Also, because the bearing contact surfaces are located within the cam, even a considerable misalignment of the outer connected ends of the helix and torsion rod with this axis would result in only a negligible angular pivoting of the cam with respect to this axis. The included angle of the conical recess 84 in cam 80 is slightly greater than the included angle of the conical bearing member 60, and the apexes of the recess and bearing member are slightly rounded off.

When the thrust bearing 60 and the bimetal helix sub-assembly have been assembled in one of the casing sections, the other casing section is placed thereon and the casing sections are rigidly connected by rivets 11 inserted in holes 13. Because the free diameter of the slit mounting tube 64 is greater than the bore formed by the recess portions 63 of cavities 62, it is necessary to compress the tube when the casing sections are assembled. This results in the mounting tube being frictionally retained against rotation in the casing. The purpose of this arrangement is to permit convenient factory calibration of the device.

Squeezing of the mounting tube 64 releases its frictional engagement in the casing and permits free rotation of the helix and cam to predetermined angular positions with respect to the switch blades 34. For example, referring to FIG. 3, assume that the switch in righthand casing 12 controls operation of a circulating fan in a warm air heating system and that the switch in lefthand casing section 10 is a high temperature limit switch which cuts off burner operation when the furnace plenum temperature reaches a predetermined high. Also, assume that the cam and the switches are in the positions shown in FIG. 3, with the switch adjustment screws 88 and in a mean range position, and that increasing plenum temperature causes the cam 80 to be rotated counterclockwise by helix 72.

In order to calibrate the device so that the fan control and high temperature limit switches are operated to an open contact position at predetermined plenum temperatures, the slit mounting tube is compressed and clamped in a compressed condition so that it will turn freely in the casing. The bimetal helix is then immersed in a bath having the temperature at which it is desired to operate the fan control switch to an open contact position. The helix and cam are then rotated clockwise until cam portion 89 causes the switch in casing section 12 to snap to an open contact position.

Following this, the helix and cam are now rotated oppositely counterclockwise from this position a predetermined amount to a second position at which it is desired to operate the high temperature limit switch in casing section 10 to an open contact position. This amount of counterclockwise rotation is determined from the known rate of rise of cam surface portion 83 and the angular movement imparted to cam 80 by helix 72 per degree of temperature change. With the cam in this second position, the switch adjustment screw 90 is now rotated until the high temperature limit switch in casing section 10 snaps to open contact position. Following this, the mounting tube is released from its compressed clamped condition and again becomes frictionally fixed in the casing.

The position of mounting tube 64 may now be rigidly fixed with respect to the casing by insertion of a cement in the counterbore formed by cavities 62, seeFIG. 4. The lever 93 on the fan switch adjusting screw 90 is provided with circularly spaced lugs 96 on the side thereof facing the casing wall, and there are circularly spaced lugs 97 on the casing wall, see FIGS. 1 and 8. The lever 93 may be swung up or down a limited amount from the position shown in FIG. 1 to turn adjustment screw 90 and vary the control point of the fan switch without interference of lugs 96 and 97. However, when the lever 93 is rotated downward sufficiently, the lugs 96 thereon will climb up on the lugs 97 on the casing and result in the considerable outward movement of the adjusting screw 90. This considerable outward movement of screw 90 moves the notched end 49 of the conductive strip 46 outward sufficiently to maintain the switch contacts closed for constant fan operation or at least maintains fan operation well below the calibrated control point.

The knurled knob 94 on the exterior end of the fa switch differential adjustment screw 88 has an arcuate recess 98 in the underside thereof, and a lug 99 projecting from the face of the casing wall enters this recess and limits the range of adjustment of screw 88, see FIGS. 1 and 9.

A snap-on cover member 100 overlies intermediate portions of the front and adjacent sides of the casing and includes offset portions 101 which enclose the heads of switch adjustment screws 88 and 90 and portions of the lever 93 and knob 94. The casing sections 10 and 12 and snap-on cover 100 are preferably molded plastic members. The cover member 100 is provided with holes 102 which cooperate with integrally cast studs 103 projecting from the side walls 16 of the casing sections. The studs 103 have rounded heads which are slightly larger in diameter than the holes 102 in the plastic cover member, requiring the holes 102 in the plastic cover member 100 to be stretched slightly as the cover member is snapped in place. The offset portions 101 of the cover member have openings 104 and 105 at one side through which the end of lever 93 and a portion of knob 94 project. The casing sections 10 and 12 are each further provided with a flange portion 106 for mounting the device on support means.

I claim:

1. In a temperature responsive control device, a casing, a cam in said casing, a coiled bimetal element supported by said casing, means operatively connecting said element to said cam whereby said cam is rotated in opposite directions by said element in response to directional temperature change, and means in said casing guiding said cam for rotation about an axis, said cam having a pair of circularly spaced peripheral cam surfaces thereon, which surfaces rise radially in opposite directions, a pair of switches mounted in said casing on opposite sides of said cam member, said switches each having two operative positions and each being biased in one of said operative positions, each of said switches being arranged to be operatively engaged by one of said cam surfaces and to be moved to the other of its said operative positions as its respective cam surface rises radially with respect thereto.

2. In a temperature responsive control for controlling the operation of a circulating fan in accordance with a furnace plenum air temperature and for cutting off furnace burner operation when the plenum air exceeds a predetermined temperature comprising a casing, a cam in said casing, an exterior bimetal element supported by said casing, means extending through a casing wall connecting said cam to said element to effect the rotation thereof in opposite directions as said element responds to directional change in temperature, said cam having a pair of circularly spaced peripheral cam surfaces thereon, which surfaces rise radially in opposite directions, a normally closed fan switch mounted in said casing on one side of said cam and a normally closed temperature limiting switch on the opposite side of said cam, each of said switches being engaged by one of said cam surfaces and being operated to an open position as the rise of its respective cam surface increases with respect thereto, whereby rotation of said cam in one direction in response to a plenum temperature increase effects opening of one of said switches and opposite rotation in response to a decrease in plenum temperature effects opening of the other of said switches.

3. The temperature responsive control device claimed in claim 2 in which said circularly spaced peripheral cam surfaces are'of identical contour and are joined at their high ends and at their low ends by peripheral dwell surfaces.

4. The temperature responsive control device claimed in claim 2 in which said switches each include a resilient switch blade having a portion thereof engaged by one of said cam surfaces to effect operation of the switch and in which means for independently adjusting said blade portion relative to its respective camming surface are included.

5. The temperature responsive control device claimed in claim 2 in which said casing comprises two identical casing sections each having side walls and an end wall and being joined at their open ends with their side walls abutting, in which the axis of rotation of said cam is parallel and coincident with the joining line of said casing sections, and in which said fan switch and said temperature limiting switch are of identical construction and are arranged identically, one in each of said casing sections.

6. The temperature responsive control device claimed in claim 5 in which said bimetal element is a coiled bimetal strip exterior of said casing and having one endthereof connected to said casing and the other end thereof connected to said cam by a torsion rod extending the wall of said casing.

7. The temperature responsive control device claimed in claim 6 having an access opening in said casing for said torsion rod, which access opening is centered on the joining line of said casing sections and has equal portions thereof formed in each of two abutting side walls, and one of said casing sections is rotated with respect to the other to provide right and lefthand casing sections with respect to said portions forming said access opening.

8. in a temperature responsive control device, a casing, a helically wound bimetal strip fixed at one end of said casing and extending to a free end, a torsion rod connected at one end to the free end of said helix and extending coaxially through said helix and into said casing through a bore therein to a free end, a rotary cam having one end thereof connected to the free end of said rod, said cam having a peripheral cam surface and an axis of rotation coincident with the axis of said helix and said torsion rod, switching means mounted in said casing operatively engaged by said peripheral cam surface, the other end of said cam having an axially aligned conical recess therein, a thrust bearing member mounted in said casing and including an axially aligned conical portion entered into said conical recess in said cam member, the apexes of said conical recess and said conical bearing portion being slightly rounded off and in contact, and the included angle of said conical recess being slightly greater than that of said conical bearing portion, and means axially biasing the apexes of said conical portion and said cam recess in contact.

9. In a temperature responsive control device, a casing, a torsion rod having a portion extending freely into said casing through a bore in the wall thereof and a portion extending exteriorly of said casing, a bimetal coil having one end connected to said casing and its other end connected to the exterior end of said rod, whereby said rod is rotated when said bimetal coil responds to temperature change, a rotary cam having one end connected to the interior free end of said rod for rotation therewith on an axis coincidentwith said rod, switching means in said casing, said cam member having a peripheral cam surface operatively engaging said switching means, a thrust bearing member mounted in said casing adjacent the other end of said cam member, one of said members having a conical recess therein and the other of said members having a conical portion entered into said conical recess, the axes of said conical portion and said conical recess being in alignment with the axis of said torsion rod and said cam, and the included angle of said conical portion being slightly less than the included angle of said conical recess and the apexes of said conical portion and said conical recess being slightly rounded off, and means axially biasing said rounded off apexes in contact.

10. The temperature responsive control device claimed in claim 9 in which said bimetal coil is a helically wound bimetal strip surrounding and extending coaxially with the exterior portion of said torsion rod, and in which the helically wound bimetal strip is stretched between its end connected to said torsion rod and its end connected to said casing, whereby the inherent resiliency of the helical bimetal coil axially biases the rounded off apexes of said conical portion and said recess into contact.

11. In a temperature responsive control device, a casing, a helically wound bimetal strip, means connecting one end of said helix to said casing and said helix extending therefrom to a free end, a torsion rod connected at one end to the free end of said helix and extending coaxially through said helix and into said casing to a free end, switching means in said casing, a rotary cam in said casing having one end thereof connected to the free end of said torsion rod for rotation therewith, said cam rotating on an axis coincident with that of said helix and said torsion rod and having a peripheral cam surface operatively engaging said switching means, said means connecting said one end of said helix to said casing comprising a longitudinally slit tube connected at one end to said helix and extending therefrom in axial alignment therewith into a bore in said casing, said longitudinally slit tube being resilient diametrally and having a free diameter larger than said casing bore and being compressed diametrally to permit insertion into said bore, whereby said tube is frictionally engaged in said casing thereby providing convenient means to selectively position said cam angularly relative to said switching means for a predetermined temperature of said helix.

12. The temperature control device claimed in claim 11 including low friction means journalling said rotary cam precisely on an axis, said means comprising a conical recess in the other end of said cam having the apex thereof slightly rounded off, and a thrust bearing member fixed in said casing having a conical portion of slightly less included angle than said recess and a slightly rounded off apex entered into said recess, said longitudinally slit tube being inserted into said casing bore sufficiently to stretch said helix thereby to axially bias said rounded off apexes of said conical bearing portion and said recess in contact, and said slit tube and said casing having cooperating abutment surfaces preventing outward axial movement of said slit tube.

13. In a temperature responsive switching device, a casing, a cam in said casing, temperature responsive means mounted on said casing operatively connected to said cam, a switch in said casing biased into operative engagement with said cam, a screw for adjusting said switch relative to said cam, said screw being threadedly engaged in said switch and extending exteriorly through a clearance hole in said casing wall and having an exterior head portion bearing against an exterior wall surface of said casing and limiting the movement of said switch toward said cam, said screw head portion and said exterior casing wall having first surfaces thereon which are in engagement during a portion of one revolution of said screw whereby said switch adjustment is effected by the pitch of said screw, and said screw head and exterior casing wall having second surfaces thereon which are engaged during another portion of said one revolution of said screw, and said second surfaces being raised with respect to said first surfaces thereby to effect a greater axial movement of said screw than is effected by the pitch thereof.

14. The temperature responsive switching device claimed in claim 13 in which said second surfaces on said screw head and said casing wall are raised sufficiently above said first surfaces to move said switch portion out of operative engagement with said cam. 

1. In a temperature responsive control device, a casing, a cam in said casing, a coiled bimetal element supported by said casing, means operatively connecting said element to said cam whereby said cam is rotated in opposite directions by said element in response to directional temperature change, and means in said casing guiding said cam for rotation about an axis, said cam having a pair of circularly spaced peripheral cam surfaces thereon, which surfaces rise radially in opposite directions, a pair of switches mounted in said casing on opposite sides of said cam member, said switches each having two operative positions and each being biased in one of said operative positions, each of said switches being arranged to be operatively engaged by one of said cam surfaces and to be moved to the other of its said operative positions as its respective cam surface rises radially with respect thereto.
 2. In a temperature responsive control for controlling the operation of a circulating fan in accordance with a furnace plenum air temperature and for cutting off furnace burner operation when the plenum air exceeds a predetermined temperature comprising a casing, a cam in said casing, an exterior bimetal element supported by said casing, means extending through a casing wall connecting said cam to said element to effect the rotation thereof in opposite directions as said element responds to directional change in temperature, said cam having a pair of circularly spaced peripheral cam surfaces thereon, which surfaces rise radially in opposite directions, a normally closed fan switch mounted in said casing on one side of said cam and a normally closed temperature limiting switch on the opposite side of said cam, each of said switches being engaged by one of said cam surfaces and being operated to an open position as the rise of its respective cam surface increases witH respect thereto, whereby rotation of said cam in one direction in response to a plenum temperature increase effects opening of one of said switches and opposite rotation in response to a decrease in plenum temperature effects opening of the other of said switches.
 3. The temperature responsive control device claimed in claim 2 in which said circularly spaced peripheral cam surfaces are of identical contour and are joined at their high ends and at their low ends by peripheral dwell surfaces.
 4. The temperature responsive control device claimed in claim 2 in which said switches each include a resilient switch blade having a portion thereof engaged by one of said cam surfaces to effect operation of the switch and in which means for independently adjusting said blade portion relative to its respective camming surface are included.
 5. The temperature responsive control device claimed in claim 2 in which said casing comprises two identical casing sections each having side walls and an end wall and being joined at their open ends with their side walls abutting, in which the axis of rotation of said cam is parallel and coincident with the joining line of said casing sections, and in which said fan switch and said temperature limiting switch are of identical construction and are arranged identically, one in each of said casing sections.
 6. The temperature responsive control device claimed in claim 5 in which said bimetal element is a coiled bimetal strip exterior of said casing and having one end thereof connected to said casing and the other end thereof connected to said cam by a torsion rod extending the wall of said casing.
 7. The temperature responsive control device claimed in claim 6 having an access opening in said casing for said torsion rod, which access opening is centered on the joining line of said casing sections and has equal portions thereof formed in each of two abutting side walls, and one of said casing sections is rotated 180* with respect to the other to provide right and left-hand casing sections with respect to said portions forming said access opening.
 8. In a temperature responsive control device, a casing, a helically wound bimetal strip fixed at one end of said casing and extending to a free end, a torsion rod connected at one end to the free end of said helix and extending coaxially through said helix and into said casing through a bore therein to a free end, a rotary cam having one end thereof connected to the free end of said rod, said cam having a peripheral cam surface and an axis of rotation coincident with the axis of said helix and said torsion rod, switching means mounted in said casing operatively engaged by said peripheral cam surface, the other end of said cam having an axially aligned conical recess therein, a thrust bearing member mounted in said casing and including an axially aligned conical portion entered into said conical recess in said cam member, the apexes of said conical recess and said conical bearing portion being slightly rounded off and in contact, and the included angle of said conical recess being slightly greater than that of said conical bearing portion, and means axially biasing the apexes of said conical portion and said cam recess in contact.
 9. In a temperature responsive control device, a casing, a torsion rod having a portion extending freely into said casing through a bore in the wall thereof and a portion extending exteriorly of said casing, a bimetal coil having one end connected to said casing and its other end connected to the exterior end of said rod, whereby said rod is rotated when said bimetal coil responds to temperature change, a rotary cam having one end connected to the interior free end of said rod for rotation therewith on an axis coincident with said rod, switching means in said casing, said cam member having a peripheral cam surface operatively engaging said switching means, a thrust bearing member mounted in said casing adjacent the other end of said cam member, one of sAid members having a conical recess therein and the other of said members having a conical portion entered into said conical recess, the axes of said conical portion and said conical recess being in alignment with the axis of said torsion rod and said cam, and the included angle of said conical portion being slightly less than the included angle of said conical recess and the apexes of said conical portion and said conical recess being slightly rounded off, and means axially biasing said rounded off apexes in contact.
 10. The temperature responsive control device claimed in claim 9 in which said bimetal coil is a helically wound bimetal strip surrounding and extending coaxially with the exterior portion of said torsion rod, and in which the helically wound bimetal strip is stretched between its end connected to said torsion rod and its end connected to said casing, whereby the inherent resiliency of the helical bimetal coil axially biases the rounded off apexes of said conical portion and said recess into contact.
 11. In a temperature responsive control device, a casing, a helically wound bimetal strip, means connecting one end of said helix to said casing and said helix extending therefrom to a free end, a torsion rod connected at one end to the free end of said helix and extending coaxially through said helix and into said casing to a free end, switching means in said casing, a rotary cam in said casing having one end thereof connected to the free end of said torsion rod for rotation therewith, said cam rotating on an axis coincident with that of said helix and said torsion rod and having a peripheral cam surface operatively engaging said switching means, said means connecting said one end of said helix to said casing comprising a longitudinally slit tube connected at one end to said helix and extending therefrom in axial alignment therewith into a bore in said casing, said longitudinally slit tube being resilient diametrally and having a free diameter larger than said casing bore and being compressed diametrally to permit insertion into said bore, whereby said tube is frictionally engaged in said casing thereby providing convenient means to selectively position said cam angularly relative to said switching means for a predetermined temperature of said helix.
 12. The temperature control device claimed in claim 11 including low friction means journalling said rotary cam precisely on an axis, said means comprising a conical recess in the other end of said cam having the apex thereof slightly rounded off, and a thrust bearing member fixed in said casing having a conical portion of slightly less included angle than said recess and a slightly rounded off apex entered into said recess, said longitudinally slit tube being inserted into said casing bore sufficiently to stretch said helix thereby to axially bias said rounded off apexes of said conical bearing portion and said recess in contact, and said slit tube and said casing having cooperating abutment surfaces preventing outward axial movement of said slit tube.
 13. In a temperature responsive switching device, a casing, a cam in said casing, temperature responsive means mounted on said casing operatively connected to said cam, a switch in said casing biased into operative engagement with said cam, a screw for adjusting said switch relative to said cam, said screw being threadedly engaged in said switch and extending exteriorly through a clearance hole in said casing wall and having an exterior head portion bearing against an exterior wall surface of said casing and limiting the movement of said switch toward said cam, said screw head portion and said exterior casing wall having first surfaces thereon which are in engagement during a portion of one revolution of said screw whereby said switch adjustment is effected by the pitch of said screw, and said screw head and exterior casing wall having second surfaces thereon which are engaged during another portion of said one revolution of said screw, and said second surfaces beIng raised with respect to said first surfaces thereby to effect a greater axial movement of said screw than is effected by the pitch thereof.
 14. The temperature responsive switching device claimed in claim 13 in which said second surfaces on said screw head and said casing wall are raised sufficiently above said first surfaces to move said switch portion out of operative engagement with said cam. 